Conventional spectroscopic imaging systems are generally based on the application of high resolution, low aberration lenses and systems that produce images suitable for visual resolution by the human eye. These imaging systems include both microscopic spectral imaging systems as well as macroscopic imaging systems and use complex multi-element lenses designed for visual microscopy with high resolution aberrations optimized for each desired magnification. Spectroscopic imaging combines digital imaging and molecular spectroscopy techniques, which can include, Raman scattering, fluorescence, photoluminescence, ultraviolet, visible and infrared absorption spectroscopies. When applied to the chemical analysis of materials, spectroscopic imaging is commonly referred to as chemical imaging. Instruments for performing spectroscopic (i.e. chemical) imaging typically comprise image gathering optics, focal plane array (FPA) imaging detectors and imaging spectrometers.
The choice of FPA detector is governed by the spectroscopic technique employed to characterize the sample of interest. For example, silicon (Si) charge-coupled device (CCD) detectors, a type of FPA, are typically employed with visible wavelength fluorescence and Raman spectroscopic imaging systems, while indium gallium arsenide (InGaAs) FPA detectors are typically employed with near-infrared spectroscopic imaging systems. A variety of imaging spectrometers have been devised for spectroscopic imaging systems. Examples include, without limitation, grating spectrometers, filter wheels, Sagnac interferometers, Michelson interferometers and tunable filters such as acousto-optic tunable filters (AOTFs) and liquid crystal tunable filters (LCTFs).
The efficiency of the of imaging spectrometers is also a function of the system-specific noise caused by background light, room temperature, the wavelength of the scattered light and the electro-mechanical or optical intangibles associated with the spectrometer. For example, the LCTF has a wavelength dependent transmission modulation which affect's the accuracy and the efficiency of measuring sharp Raman bands with weak Raman scatterers. Experiments with certain LCTF devices show complicated interactions arising in the material and structure of the imaging devices produce a spatial and spectral modulation of light coming through the imaging device. The modulation produces an apparent background signal that is not uniform and masks the real signal.
Virtually all spectral imaging devices depend on the optical properties and transmission of light through one or more optical devices in order to produce the desired filtering effect. Such devices also have complex internal configuration which affects transmission of light through the device. Although the imaging filters are designed to minimize such aberrations, residual effects remain which limit the accuracy of the device and requiring the additional step of calibration prior to imaging the sample. However, implementing such sequential steps during examination of certain in vivo biological samples is inefficient, impractical and at times, impossible.